The present invention relates to the treatment of lithographic grade aluminum webs and other substrates for use in the production of printing plates. The aluminum web is coated with a light-sensitive compound or composition by conventional methods using conventional coatings to obtain the finished lithographic plate.
Lithographic plates which have a coating of a light-sensitive material adherent to an aluminum base sheet are well known. When the light-sensitive composition is applied by the manufacturer, the plate is referred to as a "presensitized plate". If the light-sensitive material is applied to the base by the lithographer or trade plate maker, the plate is referred to as a "wipe-on" plate. Depending on the nature of the photosensitive coating employed, the treated plate can be utilized to produce the image to which it is not exposed directly, in which case it is termed a positive acting plate, or to produce an image which is complementary to the one to which it is exposed, in which case it is termed a negative acting plate. In either case, the image area of the developed plate is oleophilic and the non-image area is hydrophilic.
In coating a metallic plate with the light-sensitive material, it is highly desirable initially to provide the metal with a hydrophilic surface to which the light-sensitive coating adheres and which becomes the ink repulsive non-image area upon removal of the unconverted or unhardened light-sensitive material. Many methods of producing such hydrophilic surfaces on metallic plates for planographic printing purposes are known and are described, for example, in U.S. Pat. No. 3,891,516 to Chu, which is hereby incorporated by reference.
Lithographic plates have been grained in a variety of ways, including mechanically by rubbing with an abrasive, by sandblasting and by wire brushing, and chemically by treatment with various materials. If desired, according to Chu, the plate can thereafter be mildly etched in a warm alkaline solution and rinsed.
Anodized photographic and lithographic plates, particularly aluminum plates, are known and commercially available. In general, such plates are made by pumice slurry graining, chemically etching aluminum, or electrolytic etching of a clean aluminum sheet with alternating current, usually in a hydrochloric or nitric acid solution, to grain or etch the surface. Chu found that superior results could be achieved by pre-graining the aluminum base sheet with a wet mass of fine, hard, abrasive particles, preferably pumice, followed by direct current anodization in an acid, preferably sulfuric acid, electrolytic solution. The conventional way for anodizing an aluminum web involves connecting the anodic terminal for the DC current to a metallic roller which supports the aluminum web before it enters the electrolyte. When the DC current passes through the electrolysis tank, the aluminum is anodized and an oxide layer, the so-called anodic oxide, forms on its surface.
The thin cover layer of aluminum oxide formed on the aluminum sheet following etching and anodization is usually filled with a so-called sealing chemical and then coated with a light-sensitive material.
When employed, the common lithographic plate etching or whitening steps today utilize alkali-type etchings, phosphoric acids and acid fluorides. Each of these systems has its individual advantages and disadvantages involving process control, etching speed, etching latitude and general process efficiencies. In general, it has been found that too little etching results in a residue of imbedded pumice on the topographic surface, reduces shelf life and reduces the latitude of developability. On the other hand, over etching leads to undesirable grain directionality as well as a reduction in press life.
It is the object of this invention to provide a new lithographic plate etching or whitening procedure which provides advantages with regard to process control etching speed, etching latitude, efficiency and which minimizes the number of waste products and compositions which must be environmentally treated under government regulations which have become more complicated and more strict over the past several years.
This and other objects of the invention will become apparent to those skilled in the art from the following detailed description.